1. Field of the Invention
The invention relates to hybrid electric vehicle powertrains with a powersplit operating mode.
2. Background of the Invention
It is known design practice to provide hybrid electric vehicle powertrains for automotive vehicles wherein power delivery to the vehicle traction wheels takes place through a single ratio powersplit gearing system. An example of a powertrain of this type is disclosed in U.S. Pat. No. 6,994,360, which is assigned to the assignee of the present invention. Another example is disclosed in U.S. Pat. No. 5,791,427. In powertrains of this type, an electric power source comprising a motor-generator, a battery and a traction motor function with an engine power source seamlessly as they deliver torque through a gear system to the vehicle traction wheels. The battery acts as an energy storage medium for the generator and the motor. The generator can be driven by the engine in a series hybrid powertrain arrangement, and it can act as a motor in a generator drive mode.
In a powersplit operating mode, one power flow path is mechanical and the other power flow path is electrical. Power is distributed from the engine to the generator, which in turn provides electrical power to the motor. The motor drives the output shaft of the transmission through the transfer gearing.
When the powertrain is in a powersplit operating mode, engine power is divided into two paths by controlling generator speed.
During a powersplit operating mode, the powertrain can deliver only forward driving power to the traction wheels since there is no reverse gear. The generator requires either a generator speed control or a generator brake to effect engine torque delivery for forward drive. When the motor is used as a power source, it draws power from the battery and propels the vehicle independently of the engine in forward drive, as well as in reverse drive.
This characteristic of a single ratio powersplit powertrain may limit vehicle applications with which such a powertrain could be used. Reverse drive performance for some vehicles, such as large utility vehicles or heavy-duty trucks, require increased reverse drive torque. Further, a single ratio powersplit powertrain may not be capable of providing required forward drive gradeability in some vehicle applications, such as utility vehicles or trucks. It also may have limitations on its ability to provide trailer-towing performance.
The engines typically used in hybrid electric powertrains for large vehicle applications are larger than engines used in hybrid electric powertrains for small road vehicles. Further, they may require increased motor and generator sizes.